In the context of treatment development for diseases such as neurodegenerative diseases and brain trauma, drug access and availability to the brain are still unmet needs. Indeed, the brain is isolated from the systemic blood flow by a structure called the blood-brain barrier (BBB). The BBB is mostly composed of cerebral endothelial cells that dynamically interact with the neighbouring cells: astrocytes, pericytes, perivascular microglia and neurons. The three major functions of the BBB are the creation and maintenance of ionic homeostasis for neuronal functions, supply of the central nervous system (CNS) with nutrients, and protection from toxic injuries or some infectious agents. The delivery of therapeutic substances to the brain has to overcome the BBB, and turn it into an entry gate. Despite efforts to overcome the junctional or efflux barriers or to circumvent the BBB, most of the newly developed neuropharmaceuticals fail due to poor CNS pharmokinetics.
In the context of delivering molecules across the blood brain barrier (BBB), a type of highly soluble carriers derived from camelid heavy chain-only antibodies (HcAbs) was described. The variable heavy chain domains (VHH) of these antibodies show antigen specificity and affinity similar to conventional antibody constructs consisting of light and heavy chain heterodimers, and display a smaller size of roughly 15 kDa. A VHH is also designated “VHH fragment” to reflect that it is a portion of an antibody. VHH were shown, in specific contexts, to cross the BBB. Although for the time being no specific structural characteristic of VHH were determined to convey permeability across the BBB, an important parameter seems to be the isoelectric point (pI): permeable VHH are thought to usually be basic (especially with a pI higher than 8.5). In addition, size is considered an important factor of VHH permeability and transport capacity and in particular its low molecular weight monomeric structure is considered to contribute to these capacities. It was contemplated that targeting specific cell-surface antigens might induce endocytosis and improve the permeability of such VHH across the BBB. It was also contemplated to use a VHH specific for a brain antigen as a vehicle to target a cargo molecule to a specific site in the brain, across the BBB (Li et al., 2012, WO 2010/004432 A1, U.S. Pat. No. 8,460,888 B2). Such vehicle-cargo constructs, however, may have reduced efficiency or reduced application scope, in particular due to their binding to the brain antigen, which might result in sequestration of a majority of the cargo molecule away from its site of action.